Depends on what age of chickens we are talking about, what gender in some cases, what feed they are getting including treats, and your individual setup. It could be over crowding, too low of protein from feed that isn't age appropriate or too many treats, not enough feed (chickens should be able to eat 24/7 on something), confined space with nothing to do, too many roosters, or it could start with pecking an odd feather or toes with chicks and then moving on to doing more damage. Each of those problems has a different solution which also depends on age.